U.S. Pat. No. 4,757,747 discloses an electrohydraulic system that includes a plurality of electrohydraulic devices, specifically a plurality of actuator-controlling servo valves, individually controlled by on-board microprocessor-based control electronics. Each individual device controller is connected to and addressable by a master controller for coordinating operation among the various devices. Copending U.S. application Ser. No. 07/412,211, filed Sept. 25, 1989 and assigned to the assignee hereof, discloses a variable displacement hydraulic pump with addressable on-board microprocessor-based control electronics, pump sensors and pump-control servo valve packaged in a single assembly. In each disclosure, the servo valve is driven by a pulse width modulated valve control signal generated in the microprocessor-based electronics. U.S. Pat. Nos. 4,502,109 4,581,699, 4,612,489, 4,625,622, 4,643,074, 4,651,272, 4,714,005, 4,741,159 and 4,744,218 disclose systems suitable for implementation in microprocessor-based digital control of electrohydraulic devices, such as servo valve-controlled actuators and pumps.
Digital servo-valve control, particularly on-board microprocessor-based control, has thus enjoyed substantial acceptance and success. However, some problems which inhere in digital control techniques remain to be addressed. Resolution of the pulse width modulated valve control signal, which must be significantly higher than the natural frequency of the valve to avoid fatiguing the servo valve's torque motor, typically does not approach that of the valve itself. For example, at a pulse width modulation frequency 2,000 Hz (and thus a period of 500 microseconds), and at a microprocessor clock period of one microsecond, the highest available PWM resolution is one part in 250 for zero flow to full flow in each direction, as compared with a valve resolution of better than one part in 1,000 for flow in one direction and typical of current servo valve designs.
A further problem that reduces available PWM accuracy and resolution is associated with timer interrupt latency inherent in current microprocessor-based control schemes. That is, transitions of the pulse width modulated signal from high to low and low to high are handled by a pulse width timer interrupt routine in software. Depending upon the instructions being executed when a pulse width timer interrupt occurs, four to seven microseconds can elapse before the software interrupt routine is executed. There is thus a quasi-random timing error of up to three microseconds, which can be viewed as noise on the PWM signal that adversely affects system performance, particularly in high response systems.
It is therefore a general object of the present invention to provide a system and method for generating a pulse width modulated signal to control devices, such as electrohydraulic equipment, that address the foregoing problems in the art. A more specific object of the present invention is to provide a system and method of the described character that improve resolution of the pulse width modulated control signal to at least a resolution of one part in 1,000, which is typical for electrohydraulic servo valves of current design technology. Yet another object of the invention is to provide a system and method for obtaining high resolution pulse width modulation that may be implemented in current microprocessor-based systems of the described character through modification of control software and without any change in existing hardware.